The present invention relates to a damper valve mounted on a vehicle and provided between a hydraulic pump and a hydraulic actuator, and a hydraulic power steering apparatus using the damper valve.
Generally, a hydraulic power steering apparatus which assists steering power by oil pressure is commonly used as a steering apparatus for a vehicle. The hydraulic power steering apparatus supplies hydraulic oil discharged from a hydraulic pump to a hydraulic actuator such as a hydraulic cylinder and a hydraulic motor to provide steering assist power. A hydraulic control valve for controlling supply of the hydraulic oil to the hydraulic actuator according to a steering direction and a steering resistance is interposed between the hydraulic pump and the hydraulic actuator.
In the hydraulic power steering apparatus of this kind, such a phenomenon that steered wheels excessively vibrate due to parts precision of the steering system and the vibration transfers to the steering wheel through, for example, a piston rod (rack rod), or a so-called shimmy phenomenon may occur. To suppress such a shimmy phenomenon, such a measurement is employed that a one-way valve is provided inside the hydraulic control valve to seal the hydraulic cylinder and the hydraulic cylinder functions as a damper against the vibration.
However, in the case that the hydraulic cylinder functions as the damper described above, especially when the sharp turn is made to avoid danger, flow of the hydraulic oil to be returned to the hydraulic control valve from one of oil chambers is stopped by the one-way valve, and the steering assist power is lowered so that the problem occurs in that the heavy load applied to the steering wheel. To avoid this, damper valves are provided for respective hydraulic circuits connecting the hydraulic control valve and left and right oil chambers, respectively.
Such a damper valve is disclosed in JP-A-2001-158369, and as shown in FIG. 14, a valve sleeve 102 is disposed within a hollow casing 101, and the interior of the casing 101 is partitioned by the valve sleeve 102 into a first chamber 103 and a second chamber 104. The first chamber 103 communicates to a hydraulic control valve side through a first port 105 and the second chamber 104 communicates to a hydraulic cylinder side through a second port 106.
Such a damper valve is disclosed in JP-A-2001-158369, and as shown in FIG. 9, a valve sleeve 102 is disposed within a hollow casing 101, and the interior of the casing 101 is partitioned by the valve sleeve 102 into a first chamber 103 and a second chamber 104. The first chamber 103 communicates to a hydraulic control valve side through a first port 105 and the second chamber 104 communicates to a hydraulic cylinder side through a second port 106.
Inside the valve sleeve 102, an one-way valve 107 for allowing the hydraulic oil to flow from the first port 105 to the second port 106 and restrict the hydraulic oil from flowing in the reverse direction is provided. A plurality of passage 109 for communicating the first chamber 103 to the second chamber 104 are formed through a peripheral wall portion of the valve sleeve 102. A spool 110 capable of separating from an end surface of the valve sleeve 102 is provided at the first chamber 103, and is urged toward the end surface of the valve sleeve 102 by a spring 111. A valve member 112 for closing the passage 109 is provided between the spool 110 and the valve sleeve 102, The valve member 112 is annular shape and made of thin metal plate, and is capable of being elastically deformed by pressure of the hydraulic oil flowing to the hydraulic control valve side through the passage 109 and the first chamber 103.
With the structure of the damper valve, if the piston rod of the hydraulic cylinder vibrates right and left due to the vibration of the steered wheels, the valve member 112 is elastically deformed in response to the vibration by pressure of the hydraulic oil to flow to the first chamber 103 from the second chamber 104 through the passage 109. Thereby, The passage 109 is opened and the hydraulic oil flows and be returned to the hydraulic control valve side through the first chamber 103 and the first port 105. At this time, since the passage 109 is narrowed by the valve member 112, the damper effect is applied and the vibration from the steered wheels is suppressed from transferring to the steering wheel through the piston rod of the hydraulic cylinder.
When a driver makes the sharp turn, since a large quantity of hydraulic oil is returned to the hydraulic control valve side from the oil chamber of one of left and right hydraulic cylinder, the valve member 112 is elastically deformed in large amount and is widely separated with the spool 110 from the valve sleeve 102 against the urging force of the spring 111. Therefore, the passage is widely opened and the large quantity of the hydraulic oil is returned to the hydraulic control valve side. As a result, the problem in which a heavy load applied to the steering wheel can be prevented.
Incidentally, in the damper valve of this kind, if the valve member 112 is stuck to the valve sleeve 102 due to oil shortage, the driver feels a certain load at the steering until the valve member 112 is separated from the valve sleeve 102 and it may influence the steering feeling. Thus, a plurality of recess portions 113 are formed on a contact surface of the valve sleeve 102 with the valve member 112 and prevent the valve member 112 from the hard sticking to the contact surface.
In the above conventional hydraulic power steering apparatus, in the case the amount of the hydraulic oil returned from the hydraulic cylinder to the hydraulic control valve is a small amount and the pressure is under a predetermined pressure, the valve member 112 is not deformed and the passage 109 is closed, and at the time that the flow rate of the hydraulic oil increases and the pressure exceeds the predetermined pressure, the valve 112 is elastically deformed and the passage 109 is opened. Due to this action, the load variation occurs at the time of the valve member 112 being opened, and the driver feels ON/OFF feeling and it may gives bad influence to the steering feeling.